1. Field of the Invention
The present invention relates to a high transmittance glass sheet of soda-lime-silica glass manufactured mainly by a float process. More specifically, this invention relates to a high transmittance glass sheet that allows the formation of nickel sulfide (NiS) in a process of melting a glass raw material to be suppressed effectively.
2. Related Background Art
In methods of manufacturing a soda-lime-silica glass sheet such as a float process and a roll out process, the following problem may arise. That is, in a process of melting raw materials in a furnace at a high temperature near 1,500xc2x0 C., metal particles of stainless steel or the like containing nickel (Ni) may be mixed into the raw materials. The metal particles may react with sulfur (S) in salt cake (Na2SO4) included in the raw materials. As a result of this reaction, nickel sulfide (NiS) is formed as minute foreign matter in glass products. NiS particles are present at a minimal rate of about one per a little over 10 tons of glass products, and are of an extremely minute spherical form having a diameter as small as about 0.3 mm or less. Therefore, it is difficult to detect NiS particles on production lines.
Some soda-lime-silica glass sheets are tempered to be used for buildings, vehicles, cover glass plates for solar cell panels, solar water heaters or the like. In a tempering process, a glass sheet is heated to a temperature near the softening point (about 600xc2x0 C.) of the glass sheet. Then, the glass sheet is quenched so that compressive stress layers are generated in surfaces of the glass sheet.
When NiS is contained in a tempered glass sheet, NiS is present in an xcex1 phase that is stable at about 350xc2x0 C. or higher, and undergoes phase transition with the lapse of time to a xcex2 phase that is more stable at room temperature. This phase transition causes NiS particles to expand in volume. As a result of this, micro cracks may appear in the vicinity of the NiS particles. Inside the tempered glass sheet, a tensile stress layer exists, having a thickness of about two-thirds that of the glass sheet. When cracks appear in the tensile stress layer, the cracks run rapidly to cause spontaneous fracture of the tempered glass sheet.
To prevent such spontaneous fracture of a tempered glass sheet, so-called soaking has been known. In this method, tempered glass sheets are heated to 300xc2x0 C. or lower in a furnace (soaking furnace). Then, the tempered glass sheets are maintained in the furnace for a predetermined time, so that NiS undergoes phase transition from an xcex1 phase to a xcex2 phase. This forces the tempered glass into breakage. In this manner, defective glass products containing NiS are eliminated.
However, operations such as the soaking in which heat treatment is mainly performed cost considerable energy and time, thereby causing an increase in manufacturing cost. This also is a serious hindrance to shortening of delivery times and an improvement in productivity. Further, defective products are eliminated in the soaking, thereby causing a decrease in product yield.
JP 9(1997)-169537 A discloses a method of manufacturing a soda-lime-silica glass in which 0.01 to 0.15 wt. % of a zinc compound such as zinc nitrate and zinc sulfate is added to raw materials, thereby allowing the formation of NiS to be suppressed.
Meanwhile, there has been a growing demand that a high transmittance glass sheet, more specifically, a glass sheet having a light color and a high transmittance be used for an interior glass, a showcase, a display case, a high transmittance non-colored window glass, a high transmittance non-colored mirror, a glass substrate for a solar cell panel, a cover glass plate for a solar cell panel, a solar water heater, a material for a high solar-heat transmittance window glass, and a flat display substrate glass such as a front panel or the like. However, no high transmittance glass sheet has been known so far that is suitable for industrial mass production.
A high transmittance glass sheet according to the present invention is formed of a soda-lime-silica glass composition containing, expressed in wt. %, less than 0.020% of total iron oxide and 0.006 to 2.0% of zinc oxide. In this specification, total iron oxide denotes an amount of iron oxide in terms of Fe2O3. All of the iron in the composition is counted as Fe2O3, even if it exists as FeO.
The soda-lime-silica glass composition constituting the high transmittance glass sheet according to the present invention contains less than 0.020 wt. % (less than 200 ppm) of total iron oxide. By maintaining the content of total iron oxide at a low level as described above, it is made easier to obtain a high transmittance glass sheet having, on a 4.0 mm thickness basis, a solar radiation transmittance of 87.5% or higher. Preferably, total iron oxide is contained in an amount of not less than 0.005 wt. % as will be described later.
For the effective suppression of NiS formation in a soda-lime-silica glass composition containing less than 200 ppm of total iron oxide, zinc oxide should be contained, in terms of ZnO, in an amount of not less than 0.006 wt. % (not less than 60 ppm). The addition of zinc oxide does not cause an increase in light absorption in the visible light region. It has been found to be desirable for the suppression of NiS formation that the content of zinc oxide be increased in inverse proportion to the content of total iron oxide. When the content of total iron oxide has a value near 200 ppm, it is required that ZnO be contained in an amount of not less than 60 ppm. When the content of total iron oxide is 50 ppm, preferably, ZnO is contained in an amount of not less than 180 ppm. More preferably, when the content of total iron oxide has a value near 200 ppm, ZnO is contained in an amount of not less than 100 ppm, and when the content of total iron oxide is 50 ppm, ZnO is contained in an amount of not less than 300 ppm.
In manufacturing a high transmittance glass, in order to prevent ZnO from volatilizing during melting to damage a furnace, ZnO should be contained in an amount of not more than 2.0 wt. % (not more than 20,000 ppm). In the case where a float bath is used for forming a glass sheet, in order to prevent ZnO that has volatilized and condensed in the float bath from dropping onto a glass ribbon to form a defect, ZnO is used desirably in an amount of not more than 5,000 ppm, and more desirably in an amount of not more than 1,000 ppm.
This problem, which is caused by dropping of a condensed material that has volatilized, does not occur in the case where a glass sheet is manufactured, instead of using the float bath, for example, by a roll out process in which molten glass is rolled using a roller with an uneven (a predetermined pattern) or an even surface, and by a process in which molten glass that has been allowed to pass through a slit or overflow from a melting tub is drawn.
Thus, as shown in FIG. 1, where an x-coordinate axis indicates the content of the total iron oxide expressed in ppm and a y-coordinate axis indicates the content of the zinc oxide expressed in ppm, the glass composition has contents of the total iron oxide and the zinc oxide whose values fall preferably within a range defined by a square ABCD formed by connecting Point A (200, 60), Point B (200, 20,000), Point C (50, 20,000), and Point D (50, 180) in this order, more preferably within a range defined by a square Axe2x80x2BCDxe2x80x2 formed by connecting Point Axe2x80x2 (200, 100), Point B (200, 20,000), Point C (50, 20,000), and Point Dxe2x80x2 (50, 300) in this order, and most preferably within a range defined by a square Axe2x80x2Bxe2x80x2Cxe2x80x2Dxe2x80x2 formed by connecting Point Axe2x80x2 (200, 100), Point Bxe2x80x2 (200, 5,000), Point Cxe2x80x2 (50, 5,000), and Point Dxe2x80x2 (50, 300) in this order.
The present invention also provides a method of suppressing formation of nickel sulfide in a high transmittance glass sheet having a solar radiation transmittance of 87.5% or higher and/or a visible light transmittance of 90.0% or higher on a basis of a 4.0 mm thick glass sheet. In the method, a glass raw material is prepared so that a content of total iron oxide in terms of Fe2O3 is less than 0.020 wt % and a content of zinc oxide is 0.006 to 2.0 wt. %, and the glass raw material is melted.
The content of zinc oxide required to suppress the formation of nickel sulfide particles in a glass composition increases as the content of total iron oxide is decreased when the content of the total iron oxide in the glass is in the range of 0.006 to 0.060 wt. %. Since zinc oxide materials are costly compared with other raw materials, it would be cost effective to use zinc oxide in the least possible amount required to suppress the formation of nickel sulfide particles. Therefore, in manufacturing soda-lime glasses successively, when the content of total iron oxide in a glass composition is decreased over time, preferably, the content of zinc in the glass composition is increased accordingly within the range of 0.006 to 0.50 wt. % (60 to 5,000 ppm). Conversely, when the content of the total iron oxide in the glass composition is increased over time, preferably, the content of zinc in the glass composition is decreased accordingly in the above range.
Examples of zinc compounds for zinc oxide (ZnO) that should be added to a raw material include an inorganic zinc compound such as zinc nitrate (Zn(NO3)2.6H2O), zinc sulfate (ZnSO4.7H2O), a zinc halide (e.g. zinc fluoride (ZnF2.4H2O), zinc bromide (ZnBr2), zinc chloride (ZnCl2) and zinc iodide (ZnI2)) and zinc phosphate (Zn3(PO4)2.4H2O); and an organic zinc compound such as zinc benzoate (Zn(C6H5CO2)2) and zinc acetate (Zn(CH3CO2)2.2H2O). Although these zinc compounds have substantially the same effects, it is most preferable to use at least one selected from zinc nitrate and zinc sulfate from the viewpoint of cost effectiveness or the like.